The goal of these studies is to learn how the innervation of muscle is regulated. I will use intracellular recording, immunohistochemistry, and video-microscopy to explore the basis of selective formation of synaptic connections in skeletal muscle. The location of cues that direct the selective reinnervation of axolotl limb muscles will be investigated. Chemical and surgical methods will be used to destroy particular components of muscles such as their myofibers or extracellular matrix; the capacity of treated muscles for selective reinnervation by appropriate motoneurons will then be evaluated. The way in which muscles acquire the recognition cues that allow them to be selectively reinnervated will also be investigated by testing whether a muscle's innervation preference is altered by regenerating in a novel site, or being innervated by foreign motoneurons after regeneration. In addition, monoclonal antibodies will be made to two different axolotl muscles which are selectively reinnervated, using an immunosuppression protocol that enriches for antigenic determinants present on only one of the muscles. Antibodies that bind differentially to these two muscles will be used in future experiments to help elucidate the nature and location of antigens that distinguish the two muscles. The capacity of mammalian limb muscles for selective reinnervation will be studied by cross-innervation of muscles with foreign nerves containing axons from a range of spinal segments. Retrograde tracing techniques will be used to evaluate whether positionally appropriate motoneurons reinnervate muscles preferentially. Neuromuscular junctions (NMJs) cross-innervated by foreign axons will be examined in soleus muscles of living mice by repeatedly viewing the same NMJs in vivo over a period of several weeks or months; their stability will be compared with that of normal NMJs. The precision of reinnervation of individual endplates will be compared for foreign axons cross-innervating a muscle and for native axons reinnervating their original muscle fibers. These studies will help understand how synaptic connections are made preferentially between certain motoneurons and particular muscles. This work has potential relevance to the regeneration of synapses after injury and to the development of specific synaptic connections during development.